This invention relates generally to the field of electronic filters. More particularly, in certain embodiments this invention relates to a feed forward method and apparatus for providing a notch filter in an RF amplifier.
Radio transceivers, by definition, have both a transmitter and a receiverxe2x80x94often in the same physical package and using a common power supply, antenna and other circuitry. Due to nonlinearities in the amplification process and leakage signals from the receiver, amplifiers such as radio frequency (RF) power amplifiers can produce undesirable spectral components at the output, often including substantial energy at the receiver""s receive frequency. These undesired spectral components can create interference at a receive frequency of the associated receiver (i.e., in a transceiver) and may violate federal regulations. Feed forward compensation has been used in amplifiers to help linearize the amplifiers to compensate for such nonlinearities.
A conventional arrangement of a feedforward amplifier is depicted in FIG. 1 as amplifier 50. The power amplifier 54 in this case amplifies the RF input signal applied to its input to produce an output signal which passes through delay 58. The output of amplifier 54 is also passed through a pad 62 to reduce its magnitude. The input signal is also passed through a delay 66 and is subtracted from the output of the pad 62 to produce an error signal that represents the signal that should be removed from the output of amplifier 54 in order to linearize the amplifier 54""s amplification. This error signal is then amplified by error amplifier 72 and subtracted at element 76 from the output signal from delay element 58. The error amplifier provides enough gain so that the amount of signal subtracted at 76 approximately matches the output level of the corresponding frequency content emerging from the delay 58. In this manner, the RF output signal from the feedforward amplifier 50 is linearized and the spurious content of the RF output is reduced substantially.